public class Text7 {
    // 1314.矩阵区域和
    public int[][] matrixBlockSum(int[][] mat, int k) {
        int m = mat.length, n = mat[0].length;

        int[][] dp = new int[m+1][n+1];
        for(int i = 1; i <= m; i++){
            for(int j = 1; j <= n; j++){
                dp[i][j] = dp[i-1][j] + dp[i][j-1] - dp[i-1][j-1] + mat[i-1][j-1];
            }
        }

        int [][] ret = new int[m][n];
        for(int i = 0; i < m; i++){
            for(int j = 0; j < n; j++){
                int x1 = Math.max(0,i-k) + 1,y1 = Math.max(0,j-k) + 1;
                int x2 = Math.min(m-1,i+k) + 1,y2 = Math.min(n-1,j+k) + 1;
                ret[i][j] = dp[x2][y2] - dp[x1-1][y2] - dp[x2][y1-1] + dp[x1-1][y1-1];
            }
        }

        return ret;
    }
}
